Referring now to FIG. 1, there is shown a schematic diagram of an electrochemical fatigue sensor (EFS) device 10, in accordance with the prior art. EFS device 10 can be used to implement a non destructive fatigue crack inspection method for determining if inspected fatigue cracks are actively growing. For example, EFS device 10 may be applied to a fatigue critical location on a laboratory specimen or structure to be inspected. EFS device 10 consists of an electrolyte 12, sensor 14, and a potentiostat (not shown) for applying a constant polarizing voltage between the structure (substrate 16) and the sensor 14.
EFS device 10 works on electrochemical principles. The structure is anodically polarized to create a protective, passive film on the surface to be tested. A polarizing voltage between the structure and the electrode produces a DC base current in the cell. If the structure being interrogated by the EFS undergoes a cyclic stress, then the current flowing in the cell fluctuates in a complex relation to the variation of the mechanical stress state. Thus, an AC current is superimposed on the DC base current. Depending on the material of the structure and the loading conditions as well as the state of the fatigue damage in the structure, the transient current of the cell provides information on the status of the fatigue damage.
The electrochemical conditions imposed during EFS interrogation of a structure are designed to induce a stable passive oxide film on the surface of the material. During cyclic loading, the fatigue process causes micro plasticity and strain localization on a very fine scale. The interaction of the cyclic slip and the passivating process causes temporary and repeated alterations of the passive films. These alterations, including dissolution and repassivating processes, give rise to transient currents.
The EFS transient currents are complex, involving cyclic changes in the electrical double layer at the interface of the metal and the EFS electrolyte, generally possessing the same frequency as that of the mechanical stress, but having a complex phase relationship depending on the specific metal interrogated. In addition, the disruption of the oxide films on the metallic surface by the cyclic slip causes an additional component of the transient current which has double the frequency of the elastic current because plasticity effects occur during both the tensile and compressive parts of the cycle. As fatigue damage develops with accumulated cycles and cracks form, the cracks induce localized plasticity at different parts of the fatigue cycle from those in which the background micro plasticity occurs and in which cracks have not yet formed. The crack-induced plasticity thus introduces higher harmonic components into the transient EFS current. Analysis and calibration of these various current components allow the fatigue crack growth to be determined.
Existing EFS devices, such as that shown in FIG. 1, suffer from numerous drawbacks. For example, known EFS devices are cumbersome to attach to a substrate and fill with electrolyte. Known EFS devices also suffer from poor sensitivity, and the signal processing techniques for analyzing EFS signals generated by such devices also appear to be inadequate. The present invention addresses such shortcomings in the prior art.